Accelerometer apparatus are extensively utilized in aircraft and missile navigational systems and other applications which require such apparatus to be of relatively small size and weight with simple electrical components. Further, these apparatus must be capable of precisely measuring acceleration independent of substantial vibrational effects and wide operating temperature ranges.
One type of prior art accelerometer comprises a proof mass having a torquing coil and "pick-off" capacitors spacially arranged within a magnetic field. As an external acceleration force is applied, movement of the proof mass causes the capacitances of the pick-off capacitors to vary and a resultant current is applied via a servo circuit to the torquing coil to restore electrical and mechanical equilibrium. Current through the coil is measured via a load circuit to provide an external indication of the magnitude and direction of acceleration.
However, several problems can exist in accelerometers in accordance with the previous description. The values of the pick-off capacitors are functions of surface area and spatial air gaps between the surfaces. The spatial gaps may, of course, comprise a medium other than air with a corresponding different dielectric constant. It is extremely difficult to thus construct an accelerometer with pick-off capacitors having precisely equivalent values. Additionally, methods of manufacture and construction of other elements of an accelerometer and its associated circuitry cannot provide exact electronic circuit balance in view of the differential pick-off capacitors. One result of these problems is a dynamic bias effect when the accelerometer is subjected to normal vibrational motion. If exact balance of the electronics is not achieved, the non-equal pick-off capacitors and air gap volumes cause generation of non-zero average output signals in response to this vibrational motion. The accelerometer apparatus is then said to have dynamic bias due to nonlinearity.
A further problem with accelerometers as previously described relates to static bias. Static bias is due to various factors including residual magnetism, mechanical force effects such as static torque on apparatus components, and spring forces in the apparatus. The basic effect of static bias is to produce extraneous signals in the load circuit. Additionally, if certain types of circuitry are utilized to apply a "correction" signal to the load circuit in an attempt to overcome static bias, the signal strength may vary in accordance with differing magnitudes of acceleration forces, thereby causing additional erroneous measurements.